Monosilane, which may be referred to herein simply as silane, and which has the chemical formula SiH4, is used worldwide for a variety of industrial and commercial purposes including the production of flat-screen television screens, semiconductor chips, and polysilicon for conversion to solar cells. Due to its high purity, monosilane is emerging as the preferred intermediate for polysilicon production, where it competes with purified trichlorosilane which remains the dominant feedstock of choice due to lower overall polysilicon production costs. Further market inroads are contingent on reducing monosilane production costs—while maintaining its quality advantage, and on lowering conversion cost to polysilicon.
Most of the world's monosilane is produced using the so-called Union Carbide Process (“UCC process”), patented by the Union Carbide Corporation in 1977. In the UCC process, liquid chlorosilanes from a hydrochlorination unit are used by a monosilane production unit to make pure silane gas (SiH4). This is achieved through a sequence of distillation and catalytic redistribution reactions converting TCS into ultra-pure SiH4 and co-product STC. The co-product STC is returned to the hydrochlorination unit to be converted back to TCS.
The UCC process includes two redistribution reactors, which are used to convert TCS to SiH4. The reactor catalyst consists of dimethlyamino groups chemically grafted to a styrene based support. The support is a marcroreticular styrene-divinylbenzene copolymer. The redistribution of TCS to SiH4 occurs through the progression of three reversible equilibrium reactions as shown:2SiHCl3(TCS)SiH2Cl2(DCS)+SiCl4(STC)  1.2SiH2Cl2(DCS)SiHCl3(TCS)+SiH3Cl(MCS)  2.2SiH3Cl(MCS)SiH2Cl2(DCS)+SiH4(Silane)  3.While it is convenient to consider the transformation from TCS to SiH4 as a series of these three separate reactions, in reality, all occur simultaneously in each reactor until equilibrium is achieved. Assuming that the reaction time is long enough to satisfy the reaction kinetics and equilibrium is achieved, the product composition within each reactor is determined mainly by the composition of the feed and secondarily by reaction temperature.
The redistribution reactor performing the first reaction is called the TCS reactor because it is designed to receive a pure TCS feedstock. With a pure TCS feedstock, the equilibrium of the three reactions is such that only reaction #1 progresses measurably in this reactor. The extent of reaction under these conditions is about 20%, with the reactor product being 80% of the unreacted TCS feed and 20% products: i.e., 10% DCS and 10% STC. Due to the low first pass conversion of TCS to DCS in this TCS reactor, distillation columns are used to separate the products, recovering the more hydrogenated chlorosilanes for recycle back the TCS reactor.
A first distillation column is used to both separate the STC from the TCS in the fresh chlorosilane feed stream and separate the STC in the product from the TCS reactor. A second distillation column is used to separate the DCS from TCS in the overhead product from the first distillation column. The bottom product from this second distillation column is essentially pure TCS and becomes the feed stock to the TCS redistribution reactor.
The DCS rich, TCS lean, product exiting the top of the second distillation column becomes the feed stock to the second redistribution reactor, called the DCS redistribution reactor (“DCS Reactor”). Due to the high DCS content in this feedstock, the equilibrium of the three reactions is such that only reactions #2 and #3 progress measurably in this reactor. The extent of reactions under these conditions is such that SiH4, MCS, DCS and TCS are all present in the reactor product. SiH4 composition in the DCS Reactor product is only 12-15 mole percent at equilibrium, and thus a third higher pressure column is used to separate and purify the SiH4 from the MCS, DCS and TCS present in the DCS Reactor product. The MCS, DCS and TCS are then recycled back as a second feed to the second distillation column where the MCS and DCS are top products and feed the DCS Reactor. The TCS travels to the bottom of the second distillation column with the other TCS present in the feed stream from the first distillation column, thus increasing the amount of TCS in feed to the TCS Reactor.
In summary, a large TCS recycle loop with mass flow rate 100 times greater than that of the SiH4 product mass flow rate must pass through the TCS Reactor to convert TCS in the fresh feedstock and TCS made as a by-product of SiH4 production in the DCS Reactor to DCS. Once DCS is formed and separated from recycle TCS it becomes the feed to the DCS Reactor. A smaller DCS/MCS recycle loop whose mass flow rate is 20 times that of the SiH4 product mass flow rate must flow through the DCS Reactor to convert DCS from the second distillation column and recycled DCS and MCS from the third distillation column into SiH4.
To summarize, in the UCC process there are a total of two redistribution reactors. The first, which may be named the TCS Reactor, is located on the bottoms stream from the second distillation column. This stream is comprised almost entirely of TCS and contains de minimis amounts of DCS and STC, and is part of the TCS recycle loop. The second redistribution reactor, which may be named the DCS Reactor, is located on the overhead stream leaving the top of the second distillation column. This stream is substantially comprised of MCS and DCS, and is part of the DCS recycle loop. In normal operation, approximately 20% of TCS entering the TCS Reactor is converted to DCS and STC in roughly equal amounts, and approximately 45% to 50% of the DCS entering the DCS Reactor is converted to silane and TCS in roughly a 1:2 molar ratio.
Impurities in the crude feed stream, which comprise boron and phosphorus, are either absorbed by the redistribution catalyst, captured in filter elements, or leave with the co-product STC. The SiH4 product is of exceptionally high purity with boron and phosphorus levels at the 5-10 pptw level.
Despite the commercial success of the UCC process, it is expensive to build, maintain and operate in large part due to the large mass flow rate through the TCS recycle loop, and to a lesser extent due to the large mass flow rate through the DCS recycle loop. The present disclosure provides improvements on the UCC process and related advantages as described herein.